The present invention relates to a sewage treatment process, and more particularly to such a process for forming a solid fuel material from wastewater containing organic substances such as human wastes, and effectively utilizing the solidified fuel material.
Wastewater including drainage water discharged from homes such as human wastes is generally treated by the standard activated sludge process since the wastewater contains organic substances. In the activated sludge process, the wastewater is passed through a coarse screen to allow sludge to be gravity-settled in a settling basin, and then is passed through a finer screen for further settling sludge, after which the wastewater flows through a preliminary aeration tank into a final setting basin in which sludge is settled. The treated water separated from the sludge is passed from the final setting basin through an aeration tank into a chlorination chamber from which the sterilized water is discharged. The collected sludge is thickened by processing the same through a concentration tank and a sludge storage tank and delivering the sludge to a sludge mixing tank in which the sludge is concentrated uniformly. The concentrated sludge is then pumped from a supply tank to a filter press which separates a solid cake from the sludge. The solid cake is delivered by a belt conveyor, for example, and will be incinerated.
In the foregoing known treatment process, it takes a long time to settle the slurry containing organic substances since the organic substances have an extremely small specific gravity and a high water content. The sludge thickening is also time-consuming because the dewatering process is complex. The incineration of the solid cake requires a large amount of high-heat-capacity fuel such as coal.
The conventional sewage treatment process has therefore been disadvantageous in that it involves a number of steps since it is time-consuming for the organic substances to settle, and a complex dewatering process is required for concentrating the sludge, resulting in a high cost of sewage treatment. For increasing the dewatering rate, a dewatering agent such as slaked lime or ferric chloride, has been added. However, the use of such a dewatering agent increases the amount of inorganic substances in the treated sludge which leads to secondary pollution, and requires an increased amount of water to be used in the process.
It has recently been known to employ pulverized coal for its absorption ability or to add a high-polymer flocculation agent for facilitating and accelerating the dewatering and thickening of the sludge, and to use the produced cake finally as a secondary fuel, utilizing the latent combustion heat of the pulverized coal. However, the sludge cannot easily be settled due to the floating property of the pulverized coal. Since the sludge has to be mixed during the process, ion-based organic substances such as cation and anion are increased in amount, resulting in an additional secondary processing step. Therefore, the above known method cannot meet desired economical requirements, although the time required for dewatering the sludge can be shortened and the cake has a lowered water content.
The recent trends in treating wastewater are not confined to the processing of drainage water only, but toward economical treatment processes which effectively utilize water and solid materials produced by processing the wastewater. From this standpoint, the conventional wastewater treatment process employing pulverized coal has proven unsatisfactory in that the pulverized coal is used only as a dewatering agent, and the high cost required of treating the wastewater cannot be recovered.
There has been an attempt to employ a pressure floating method to meet the economical requirements. However, it is relatively complex to select a reagent and a selected reagent has to be consumed in an extremely large quantity. Therefore, this method is also economically problematic.